When subsea risers are subjected to even moderate currents, forces resulting from vortex shedding can result in vibrations. These vibrations can, depending on the stiffness and the strength of the riser and the welds between the riser joints, lead to unacceptably short fatigue lives. Vibrations can be particularly severe when they are of frequencies that coincide with natural resonant frequencies of the riser.
Many methods have been developed to reduce vibrations of subsea risers. Some of these methods function by modifying the boundary layer of the flow around the riser to prevent the correlation of vortex shedding along the length of the riser. Examples of such a method include the inclusion of helical strakes around the riser, axial rod shrouds and perforated shrouds. Helical strakes are often used around chimneys, and are occasionally used in subsea applications. These devices all may create additional drag, and can become ineffective if they are covered by marine life such as barnacles. They are also costly and time consuming to install.
Other methods to reduce vibrations caused by vortex shedding from subsea risers operate by stabilization of the wake. These methods include streamlined fairings, wake splitters and flags.
Streamlined, or teardrop shaped, fairings that swivel around a riser have been developed that almost eliminate the sheading of vortexes. The major drawbacks to teardrop shaped fairings is the cost of the fairing and the time required to install such fairings. Additionally, long term operation in a subsea environment can cause the shape of the fairing to change do to buildup of marine growth such as barnacles and other sea life. It is also critical for acceptable operation of these fairings that they remain rotatable. If they do not rotate, a cross-current could result in vortex shedding that induces greater vibration than the bare riser would incur. The effectiveness of the fairing is also decreased with increases in surface roughness caused by barnacles.
Wake splitters are flat plates that extend from the back of a riser parallel to the current flow direction. These wake splitters have been found to be effective to create a symmetric vortex pattern so that each vortex "sees" an image created by the rigid splitter plate giving symmetry with respect to the axis in the direction of flow. Splitter plates also stabilize the separation points, decrease the wake width and reduce drag. Splitter plates suffer from most of the same detrimental effects as teardrop shaped fairings for off-axis currents. They must therefore either be rotatable or be used only where the direction of a significant current does not vary.
Flags are similar to wake splitters, but are flexible. They are not generally as effective as wake splitters, but have the advantage that they can wrap around a riser and remain somewhat effective with varying current directions without being rotatable. Disadvantages include the probability that the flag will eventually completely wrap around the riser and therefore becomes ineffective. Flags are not commonly used in subsea applications due to this high probability of the flag wrapping itself around the riser, and because of the difficulty and expense of attaching the flag to the riser along the length of the riser.
It is therefore an object of the present invention to provide a method and an apparatus to reduce vortex induced vibrations in subsea risers that can be economically employed, and will function regardless of the relative direction of current. It is a further object to provide such a method and apparatus that will not be subject to deterioration in performance do to marine growth.